Cranes are commonly employed for the lifting and transport of objects. In many circumstances, very large cranes are operated so as to hoist large loads. Such large cranes are often used at port facilities for the transport of containers to and from cargo ships. Normally, the loading and unloading of ships at port facilities must be carried with a large amount of concern for safety. The handling of containers at port facilities must be carried out efficiently and effectively so as to prevent damage to cargo and to minimize the costs associated for the unloading and/or loading activities.
One of the common problems encountered by such cranes as such facilities is the occurrence of a "snag load". These snag loads occur whenever the crane is employed to lift weight beyond the capacity of the crane. Typically, such snag loads occur when the container is caught or attached to another object. Whenever a snag load occurs, it is important to be able to shut down the operation of the crane before the hoist ropes break or the structure of the crane becomes damaged. Under such circumstances, if the hoist motor of the crane is not stopped, then the motor and hoist system can become permanently damaged and require replacement.
Conventionally, snag load systems are in place which stop the motor upon the occurrence of a "snag load". Conventional snag load protection systems employ pressure sensors and/or transducers along the hoist rope. When pressures are encountered above a predetermined level, then the transducer will send a signal to stop the motor. Unfortunately, because of the size of the motors employed in such crane systems, the motor will not immediately stop upon receiving the signal. The continued momentum of the crane motor will still cause excessive stresses to be placed upon the hoist rope, upon the crane structure, hoist system, and upon the motor, even when the motor is turned off. As such, a need has developed so as to prevent these adverse effects from affecting the crane system adversely.
In the past, various patents have issued relating to crane protection systems. U.S. Pat. No. 4,157,736, issued on Jun. 12, 1979, to R. E. Carbert teaches an overload protection apparatus for hydraulic equipment. This system includes an overload or relief valve having a manifold to which the power lines of all of the actuators are connected. These connections are directed through check valves so that fluid within the lines will be directed only to the relief valve. Any overpressure condition operates the pilot valve so as to unload the pressure lines of the actuators and to prevent their continued operation as long as they are in the overstressed condition.
U.S. Pat. No. 4,236,864, issued on Dec. 2, 1980, to Couture et al. describes a safety control system for a boom of a crane. This system is employed so as to prevent the exceeding of the uppermost sink elevation of each articulated section of a boom and the safe swinging limits right and left of the boom.
U.S. Pat. No. 4,252,243, issued on Feb. 24, 1981, to Robinson et al. describes a crane safety device which serves to relieve hydraulic pressure from the crane motor in response to the horizontal radius of the free end of the rope so that at a given radius the motor can lift only a given maximum load. A means is provided for releasing the brake when the free end of the rope is a given distance below the crane.
U.S. Pat. No. 4,433,612, issued on Feb. 28, 1984, to Spielvogel et al. describes a safety control device for protecting hydraulically held loads against uncontrolled pressure overloading. This system employs pressure relief valves so as to prevent overload conditions from affecting the lowering and lifting cylinders of a hydraulic press.
U.S. Pat. No. 4,598,829, issued on Jul. 8, 1986, to Young et al. shows a hydraulic circuit for a crane. An unloading valve is interposed in the boom hoist circuit so as to dump the boom hoist pump directly to the reservoir when pressure in the pump circuit becomes excessive. A boom limit valve, actuated as the boom reaches both upper and lower limits, also causes the unloading valve to dump the boom hoist pump to the reservoir.
U.S. Pat. No. 5,018,631, issued on May 21, 1991, to H. J. Reimer shows a hoist device for a container crane. This system employs four hoist ropes. One end of each rope is wound on a drum to be driven in either direction to hoist or lower the container. The second end of each rope is connected to a double-acting hydraulic cylinder. The cylinders are controlled by a valve system to dampen the swaying of the load, to reset the ropes, and for skewing, trimming and lifting the load. Various valves are employed so as to prevent overload conditions.
U.S. Pat. No. 5,163,570, issued on Nov. 17, 1992, to Mundis et al. describes a load sensing device for a boom mounted on a vehicle. A worm gear arrangement is provided for articulating the boom. A hydraulic chamber on each side of the worm gear controls the operation of the boom. A thrust of the worm gear in the direction of the hydraulic chamber increases the fluid pressure in the chamber so as to cause the fluid pressure in the closed hydraulic circuit to increase. The pressure increase is sensed by a fluid pressure gage which sends a signal so as to terminate input power to the worm gear if the increase in fluid pressure is above a predetermined pressure.
British Patent No. 2,142,313, published on Jan. 16, 1985, teaches a hydraulic circuit for a crane having a plurality of pumps. If the hydraulic load on the pumps reaches a point where the motor for the pumps is near a stalling condition, pressure in a pilot passage increases to a point where an internal relief valve will shift. This connects the pilot passage to the reservoir side of the pumps so as to cause the unloading valve to shift.
British Patent No. 2,078,197, published on Jan. 6, 1982, describes a load limiting device for hydraulically operated cranes. A pressure switch senses the pressure in the hydraulic cylinder of the of the lifting arm. The switch is disconnected when the pressure reaches a predetermined maximum value.
European Patent No. 224,446, published on Jun. 3, 1987, describes a load limiting device for a hydraulic crane which includes a load sensing means incorporating two-way, two-position response valves which pick up pressure signals from the end of the hydraulic cylinder. The response valve connects with a pump and a send line through which the signal is transmitted. When the signal from the cylinder exceeds the maximum rated capacity of the crane, the valve opens and stops fluid flow to the lift cylinders.
It is an object of the present invention to provide a snag load protection system which prevents damage to the crane.
It is another object of the present invention to provide a snag load protection system that can be retrofitted to existing cranes.
It is another object of the present invention to provide a snag load protection system that maximizes the safety of the crane operation.
It is another object of the present invention to provide a snag load protection system that is easy to use, easy to install, and relatively inexpensive.
These and other objects and advantages of the present invention will become apparent from a reading of the attached specification and appended claims.